Bending of elongate members such as steerable catheters allows these devices to be used in many beneficial ways, including when performing surgical procedures. For a catheter (and similar elongate, bendable members), articulation may be achieved by tensioning a tendon (also referred to herein as pull wire) which runs the length of the device, and is typically attached at the distal end, so that it can be actuated proximally. In a device with free floating tendons, wherein the tendons are not pre-tensioned, a certain level of slack within the catheter will likely exist. To remove the slack consistently, a tensioning algorithm based on measured force on the wire may be used. However, sensor size and placement can result in noise or inaccurate readings. Likewise, in the event of a wire breakage, external force sensors may detect a drop of tension on a wire, but may be unreliable, and may increase the complexity, size, and cost for the catheter.
One alternative to a traditional strain gauge is a resistance wire tension measuring gauge, such as described in U.S. Pat. No. 4,803,888, in which the electrical resistance of a wire was found to be related to the strain on the wire. To date, however, adapting such resistance measuring wires for us as part of a steerable guidewire has not been descried and/or successfully achieved. This may be due to the relative complexity of the tendon-drive elongate members, including the need for small diameters and the difficulty in accurately delivering power to the wire(s) acting as a strain gauge and the uncertainty associated with detecting a robust change in resistance with materials useful as tendons (wires) in steerable elongate members useful for medical applications.
Thus, it would be useful to provide tendon-steerable elongate devices such as catheters in which the tension on the tendon may be easily and accurately detected. Described herein are elongate, steerable devices, such as catheters, that are configured to directly measure the wire tension, as well as reliably detect wire breakage.